One of the shortcomings of semiconductor diode lasers is that there are inherent limits on the power output obtainable from a single element diode laser. Above a certain power, the single element diode laser, no matter what its construction, will be destroyed by facet-mirror damage or by heating from the electric power dissipated within it.
One approach to circumventing these inherent limitations, to achieve output powers of 100 to 500 mW, involves coupling a line of index guided laser devices to form a phase-locked laser array. In a linear phase-locked array, 8 to 10 index-guided lasers are fabricated on the same substrate so that each has a structure identical to that of its neighbors. Illustratively, the 8 to 10 index guided laser elements extend between a pair of parallel, spaced apart, partially reflecting crystal end facets which form a Fabry-Perot Resonator for the individual laser elements in the array. A pumping current may be applied to the individual laser elements comprising the array by a common electric contact as wide as the entire array.
The evanescent optical field of each laser element in the array overlaps the neighboring laser elements. Thus, each device couples with its nearest neighbors and all the lasers in the array act together as one powerful source.
When individual index-guided lasers are coupled in a phase-locked array, they can operate in many array modes. If they couple together in phase--the so-called 0.degree. phase shift mode--the far field patterns from the individual elements interfere with each other constructively on axis and can provide a well-defined beam comprising a single lobe. If they couple so that each laser element is out of phase with its neighbors--the 180.degree. phase shift mode--the far field patterns from the individual laser elements interfere destructively on axis, generally producing a two-lobe beam. There are also complex intermediate cases, but none produces a well collimated single lobe beam except the zero degree phase shift mode.
In theory, the narrowest possible angular width of an array's beam is about equal to the wavelength emitted by the array divided by the product of the number of elements and the spacing between them. This is the beam width of a single source as wide as the whole array, limited only by the diffraction of light. For example, a 10-element array in which each element is separated 5 microns from its neighbors and which emits at a wavelength of 0.85 microns should be able to deliver up to 500 mW of power into a beam as narrow as 1 degree.
In practice, operation of a laser array in the zero degree phase shift mode to produce a single-lobe, diffraction-limited beam in the far field has been more the exception than the rule. Presently available laser arrays are more likely to oscillate in the 180.degree. phase shift mode, as this mode is less lossy than the zero degree phase shift mode and thus has a lower threshold current. In addition, the most common experimental result in phase locked arrays has been a two-lobed far field pattern similar to that obtained in the 180.degree.phase shift mode, but wherein the width of each lobe is two to three times the diffraction limited width as a result of several array modes oscillating simultaneously. Thus, presently available laser arrays are unsuitable for many applications such as optical recording and high bit rate optical communications which require single-lobe, diffraction-limited beams in the far field.
Accordingly, it is the object of the present invention to provide an array of index-guided laser elements formed on a single substrate, which array oscillates stably and reliably in the zero degree phase shift mode so as to produce a single lobe diffraction limited beam in the far field.